The apparatus of the present invention is designed to facilitate the berthing and loading of a shuttle tanker from a fixed non-compliant offshore production/storage terminal, especially in a severe arctic environment. It is desirable to allow the connected tanker to weather vane without limit to either direction around the offshore production/storage terminal. By permitting the moored vessel to weather vane around the terminal the mooring loads are greatly reduced by taking advantage of the natural sheltering provided by the terminal structure. Furthermore, the telescoping boom arrangement of the present invention provides a compliant connection between the tanker and the platform to further reduce mooring loads by permitting first order motions of the tanker. First order motions are high frequency low amplitude oscillations of the tanker due to environmental conditions.
Typically, offshore bottom founded production/storage gravity structures designed for arctic service have a general conical profile below the water level and have base diameters as large as 600 feet and more. These structures may be placed in offshore locations with water depths of 60 feet or more. Typical ice capable tankers used to load crude oil from such production/storage terminals can be as long as 1,100 feet or more. The design of a mooring system must take into account the environmental conditions anticipated when ice conditions appear as well as open water conditions. Typically open water conditions are of a dynamic nature and tend to introduce large amplitude motions while the magnitude of the applied forces is relatively small. Ice conditions introduce large amplitude low frequency loading that results in higher stresses in the structure but introduces relatively little motion.
During open water conditions mooring forces are generated due to wave, current and wind loading on the mooring vessel. During the colder months the effect of ice on the moored vessel must be considered in the design of a mooring system. Empirical methods for calculating the loading of ice on vessels transiting through and ice field are known, as indicated by G. P. Vance "A Scaling System for Vessels Modeled in Ice" Society of Naval Architects and Marine Engineers, Proceedings, ICETECH 75, Montreal, Canada, April 1975. Analytical methods for calculating ice loads on vessels transiting through a broken or unbroken ice field are also known as indicated by V. R. Milano, "Variation of Ship/Ice Parameters on Ship Resistance to Continuous Motion in Ice" and "Ship Resistance to Continuous Motion in Ice" Society of Naval Architects and Marine Engineers, Proceedings, ICETECH 75, Montreal, Canada, April 1975. These methods have to be applied to moored vessels in an advancing ice field.
Taking into consideration the ice loads for an assumed ice thickness of fifteen feet as well as wind and current loads the apparatus of the present invention has been analyzed with an expected total tensile load on the boom of 4,600 kips applied along the centerline of the boom at an angle of inclination with the horizontal plane of twenty degrees maximum. Although the moored vessel will use astern power and/or side thrusters to the extent required to keep off the terminal structure, during transient conditions the vessel may apply some rideup load to the structure. The apparatus of the present invention is suited to handle rideup or compressive loads on the order of 1,000 kips and a side load of 500 kips.
The mooring/loading system of the present invention has the capability of year around unassisted mooring, demooring and loading of shuttle tankers to a fixed production structure. The system is capable of maintaining the shuttle tanker moored during open water or ice conditions. First order vessel motions including vessel surge are compensated for by the telescoping boom arrangement. The vessel is permitted to continuously weather vane around the production structure while loading crude uninterruptedly. The mooring and loading apparatus accommodates significant variation in the height of the ship-side mooring connection above the mean sea surface, provides automatic emergency decoupling of the mooring and loading system, and further maintains the mooring boom with the loading lines clear of the sea surface. The apparatus of the present invention allows the mooring equipment to be enclosed when not in use and further provides for tracing and heating of weather exposed surfaces for de-icing purposes. The mooring system provides compensation for movement in the horizontal and vertical planes with a minimum impact on the terminal structure to which it is connected. Finally, the mooring system of the present invention allows a safe and quick mooring/demooring procedure without assistance from service boats.